I prefer the fighter/thief multiclass, but the thief kits are pretty good as well. For backstabbing the assassin kit is the best option since it reaches a x7 damage multiplier instead of the normal x5. The swashbuckler cannot backstab. Bounty hunter is an interesting option with the traps, if you're into that sort of thing.

Fighter dualed to thief (such as kensai/thief) can be powerful, but you'll miss out on the fighter high level abilities and will have to spend some time as just a pure class thief waiting for your level to catch up to the kensai.

Then there is the thief/mage combo... never really bothered with them, but they are the most powerful because arcane magic dominates this game.

And some might even like the cleric/thief.

I recommend installing the mod(s) Assassinations and/or Rogue Rebalancing.

Like Suiboon says, a thief/mage combo is the most powerful. Not only can you use invisibility spells to augment your number of backstabs, the spellcasters in BG are the most powerfull in this game anyway, so it's more like the thief skills are a nice bonus for the mage class.
So if you want to roleplay a thief, I'd go with one of the kits. Personally I played a mage/thief a few times, but in the end I played like a mage more than a thief.

I liked the bounty hunter a lot, because of the traps. But it's a bit unfair as I know what kind of encounters are waiting for me and where, so I can always place traps a little too tactical, making the game a bit too easy sometimes.

I ended up going with thief/mage because I thought they looked like fun witht he invisibility spell. I was choosing between that and a Kensai/Thief which I had heard alot of good things about.

I approve. :-) This is good choice. Thief/mage with improved invisibility is great killer.

If you want to play a thief, go assassin all the way. But I would definitely use the rogue rebalancing mod. It makes the assassin much better in my opinion and really makes the kits more unique. But I think the assassin is very cool to play.

I don't play pure thieves. Can't manage it. Try a Fighter/thief

Currently trying the swashbuckler. Meh, backstab is nice, but not required to win, and the bonuses it has makes up for the classes lack of it.

Thieves definitely require micromanagement and a more subtle approach to things if that's what you prefer. The RR mod really fleshes out thieves quite a bit without going overboard. I love backstabbing and the whole stealth thing so thieves are my favorite class to play. I recently finished a run though using the modded assassin I was talking about in an earlier post. One of the new abilities they get is called 'death attack'. You get one initial use of it at level 16. Once activated, you automatically gain a critical hit against the enemy. The target then has to make a save vs. death or is slain immediately. In higher levels you get more uses of it and the enemies take a penalty to their death save. Pretty wicked. Doesn't work against constructs, elementals, undead, and slimes. I've gone right up a to mage in stoneform and dropped them in one hit. You don't need to be in stealth or behind the target. Now the one cheesy part is that all the dragons in SoA can be death attacked so I wouldn't use it against them unless you really want an easier fight. I haven't tried to other thief changes yet but am going to soon. Peace

Fighter/Thief is probably the easiest build to play imo, though I'm not SUPER experienced (never actually done any kensai duals). I played a halfing f/t last time through. Always made my saves (halfling w/ max con) and once I got use any item I was close to being a tank. I opted for short sword/sling combo, but if I had been more melee focused I could have easily been a bonified tank I think. And you'll always have plenty of thieving points. I'm playing an elf assassin right now and that's not the case. I probably should have planned the point use better. Still, I'm having fun with him, though he is kind of hard to use (pretty fragile). I've heard that Bounty Hunters are really fun and that their special traps add a new level of strategy to the game. One day I'd like to solo with one :). Swashbuckler I've never played either. F/T is probably better in every respect.

Edit: I'm also using the Assassinations mod. I recommend it too. It's pretty well designed and implemented. Never tried Rogue Rebalancing because I think it changes too much, but the new take on the Short Sword of Backstabbing seems pretty epic (it increases your backstab multiplier by 1, sorry if that's a spoiler?)

While creating an Assassin, i noticed the starting available skill points is 145??!! Is this right? im playing after about 8 years.

EDIT: Nevermind, i forgot that characters start out with 89k XP... YOW!